Security films of the type taught in U.S. Pat. No. 3,801,183, (thin easily tearable films which incorporate a monolayer of glass microspheres and a latent image viewable in retroreflection) sometimes need to be adhered to a substrate with a pressure-sensitive adhesive rather than the preferred heat-activated type. One example is when the security film is applied to a document at various decentralized locations rather than a central location where more expensive heat-laminating equipment is available.
Use of a pressure-sensitive adhesive increases the danger of transfer of the film from one substrate to another document to falsely authenticate that document. The danger does not lie in ordinary room-temperature removal, since aggressive pressure-sensitive adhesives are available that prevent such removal of the quite flimsy security film from a substrate to which it has been applied. Also, the security film uses binder materials that are soluble in any solvent that the pressure-sensitive adhesive is soluble in, so solvent loosening of an applied security film can be prevented.
However, with certain constructions it might be possible to apply heat to a security film to soften the pressure-sensitive adhesive and allow removal of the film, and no one has previously solved that problem.